During the pumping of a liquid or suspension containing gas, using a centrifugal pump, there is a tendency for the gas to collect at the center of the pump impeller in a bubble. This can be a significant problem in the pumping of a number of liquids or suspensions, such as cellulosic fibrous material suspensions (e.g. paper pulp) particularly those having a medium consistency, that is a solids consistency in the range of about 8-15 percent. If the bubble becomes too large, it hinders the pump operation. Therefore it is necessary to in some way control and regulate the gas collection so that the pump performance can be optimized.
In U.S. Pat. Nos. 4,410,337 and 4,435,193, the disclosures of which are hereby incorporated by reference herein, methods and apparatus for discharging gas from centrifugal pumps in order to control pump capacity are described. The gas is discharged through an outlet while the pump head and the pump output are controlled by maintaining a pressure difference between the suspension inlet and the gas outlet. However when the gas is controlled in this way, the pump often does not operate at peak efficiencies, and additionally there are problems that result in view of the fact that the pulp being pumped may contain varying quantities of gas, and other variables may exist.
According to the present invention, by taking advantage of the difference in electrical conductivity between the liquid or suspension being pumped, and the gas in the bubble being formed, it is possible to sense (e.g. measure) the size of the gas bubble. The discharge of gas from the pump is then controlled so as to regulate the size of the bubble so that it does not impede pump operation. However there is no necessity, in such a circumstance, for creating a pressure differential as exists in the above-mentioned U.S. patents.
The method is practiced by utilizing a plurality of electrodes which extend through a wall of the pump defining, with the impeller, a chamber in which the gas collects. The electrodes are radially spaced from each other, and from the axis of rotation of the pump, and are operatively connected through resistances to a source of direct or alternating current. A voltage meter is placed across one of the resistors--namely the resistor of the electrode that is closest to the axis of rotation, and the output from the voltage meter can be used to automatically control a valve, or other mechanism, for facilitating the discharge of air from the bubble.
Utilizing the present invention it is also possible to measure the rpm of the impeller, which, when compared to the idle rpm of an asynchronous motor which drives the impeller, can be used to determine the power consumption of the pump. Also it is possible to control the size of the gas bubble to regulate the pump pressure head so that it is in a predetermined range, and, in connection with the speed of the impeller, the volume capacity and power consumption can be controlled so as to obtain the greatest possible operating efficiency.
It is the primary object of the present invention to provide a method and apparatus for effective control of the operation of a centrifugal pump by maintaining the size of the bubble of gas collecting in the pump chamber within a predetermined range. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.